This invention relates generally to fiber optic communications systems. This invention relates particularly to an apparatus and a method for coupling optical signals from one optical fiber into a selected one of a plurality of optical fibers.
Fiber optic telecommunications systems employ optical fibers for carrying communications signals because such systems have the capability of carrying signals on a large number of channels. Such systems are necessary for meeting the demand for telecommunications channels.
A fiber optic telecommunications system includes switching stations so that communications signals may be routed between any two locations in the network. Each switching station switches signals carried by one optical fiber to a selected one of a plurality of optical fibers for transmission to another switching station in the network.
Systems for switching optical signals between optical fibers include apparatus for placing the fibers end-to-end so that light may be coupled out of one fiber into the other. Direct coupling between single mode fibers is not practical. Therefore lenses are typically used at the ends of the fiber. A first lens expands the light beam output from one of the fibers. A second lens collects the beam output from the first lens and focuses it on the end of the second optical fiber. The use of lens elements makes the coupling device complicated, large, unstable and expensive and requires critical alignment. As the light crosses each interface between media having different refractive index, signal loss occurs due to reflections.
The switching station should be highly reliable and have low insertion loss and high return loss. Insertion loss reduces signal strength, whereas returned, or reflected, signals cause cross talk. The fiber ends must be axially and laterally aligned and must be spaced apart by only a very small distance to meet the operational requirements.
Some present fiber optic switching stations use robots to move the fiber from which a signal is to be extracted to a selected contact point where the signals is coupled into another optical fiber. Robots for this purpose are very expensive and have problems with repeatability in placing the fibers in positions where signals may be satisfactorily extracted from one optical fiber and input into another. Robots are also slow and have limitations on the number of fiber optic channels that may be used.
Other prior art switching apparatus (e.g. U.S. Pat. No. 4,896,935) uses a stepping motor to place the fibers in alignment for signal transmission. Stepping motors have the disadvantages of poor resolution, non-uniform stepping and thermal sensitivity.
Accordingly, there is a need in the art for a reliable, low loss technique for extracting an optical communications signal from one fiber and injecting the signal into a selected one of a plurality of fibers. There is also a need for a system that provides a larger number of channels than present systems.